1. Technical Field
The present invention relates to a variable resistance element, particularly to a variable resistance element having a resistance value which reversibly changes according to applied electrical signals.
2. Background Art
In recent years, along with the development of digital technology in electronic devices, there has been an increasing demand for nonvolatile memory devices having a large capacity for storing data such as music, image, or information. As a solution to meet such demand, attention is being focused on a nonvolatile memory device (hereinafter referred to as a ReRAM) which uses a variable resistance element in a memory cell, the variable resistance element having a resistance value which changes according to applied electrical signals, and the state of the resistance element being retained by the variable resistance element. This is because ReRAM has characteristics that the configuration of the variable resistance element is relatively simple and thus can be highly integrated easily, and the compatibility with the conventional semiconductor process can be effortlessly achieved.
As examples, Japanese Unexamined Patent Application Publication No. 2008-306157, Japanese Unexamined Patent Application Publication No. 2006-203178, and WO 2010/086916 each disclose a variable resistance element including two electrodes and a variable resistance layer interposed between the electrodes, the resistance state of the variable resistance layer being reversibly changeable. FIGS. 8, 9, and 10 are cross-sectional views illustrating the configurations of conventional variable resistance elements which are disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 2008-306157, Japanese Unexamined Patent Application Publication No. 2006-203178, and WO 2010/086916, respectively.
FIG. 8 illustrates the configuration of a conventional variable resistance element 800 described in Japanese Unexamined Patent Application Publication No. 2008-306157. The variable resistance element 800 has a basic structure (the upper illustration in FIG. 8) in which a variable resistance layer 806 including a metal oxide layer is interposed between a first electrode 807 and a second electrode 805. By applying a predetermined voltage across the first electrode 807 and the second electrode 805 of the variable resistance element 800 with the basic structure, a filament 806c (the lower illustration in FIG. 8) is formed as a current path (a region where the density of a current which flows across the electrodes locally increases) between the first electrode 807 and the second electrode 805. Hereinafter, the process of forming the filament 806c for the first time is referred to as an initial breakdown, and a voltage necessary for causing the initial breakdown is referred to as an initial breakdown voltage.
FIG. 9 illustrates the configuration of a conventional variable resistance element 900 described in Japanese Unexamined Patent Application Publication No. 2006-203178. The variable resistance element 900 includes a second electrode 905 having nano-needles 905a. The variable resistance layer 906 is adjacent to the nano needles 905a. The first electrode 907 is adjacent to the variable resistance layer 906. Because only the second electrode 905 is provided with the nano needles 905a which are conductive, the variable resistance element 900 has an asymmetric electrode structure. The number of nano-needles which extend on one square micron area on the surface of the second electrode 905 normally exceeds 100.
Normally, the higher the density of the nano-needles 905a, the greater the difference between the resistance values of the variable resistance layer 906 in a high resistance state and a low resistance state, each resistance value being changeable according to an applied constant voltage pulse. The performance characteristics of the variable resistance element 900 are improved by a nonuniform electric field generated by the nano-needles 905a. The electric field at the tip of the nano needles 905a is much higher than the overall average electric field. Therefore, the resistance value of the variable resistance layer 906 can be changed using a weak electrical pulse having a low voltage.
FIG. 10 illustrates the configuration of a conventional variable resistance element 1000 described in WO 2010/086916. The variable resistance element 1000 includes a substrate 1001, an oxide layer 1002 formed on the substrate 1001, a first electrode 1007 formed on the oxide layer 1002, a second electrode 1005 having a plurality of needles 1005a, and a variable resistance layer 1006 interposed between the first electrode 1007 and the second electrode 1005. Here, the variable resistance layer 1006 comprises an oxygen-deficient oxide, and includes a first metal oxide containing layer (hereinafter referred to as a “first metal oxide layer”) 1006b having a high degree of oxygen deficiency, and a second metal oxide containing layer (hereinafter referred to as a “second metal oxide layer”) 1006a having a low degree of oxygen deficiency, which is formed on the first metal oxide layer 1006b. 
The thickness t of the second metal oxide layer 1006a is greater than the height h of each of the needles 1005a. Thus, the distance between the tip of each of the needles 1005a and the first metal oxide layer 1006b is t−h and is smaller than t which is the distance between the portion of the second electrode 1005 where no needle 1005a is present and the first metal oxide layer 1006b. A plurality of needles 1005a is formed by heating the second electrode 1005. Because the electric field is concentrated on the vicinity of the tips of the needles 1005a due to the formation of the plurality of needles 1005a, a filament region, in which a resistance change phenomenon occurs, is more likely to be formed.
However, with any of the configurations of Japanese Unexamined Patent Application Publication No. 2008-306157, Japanese Unexamined Patent Application Publication No. 2006-203178, and WO 2010/086916, a filament region in which a resistance change phenomenon occurs, and the needles that define the location of the filament region are formed at random in a plane parallel to the substrate of the electrodes. In addition, the composition of the metal oxide comprised by the variable resistance layer is not uniform in a plane parallel to the substrate. Specifically, the vicinity of the side wall of the variable resistance element tends to have an adverse effect such as etching damage or oxidation in a process of forming an interlayer insulating layer more severely than the central portion of the variable resistance element has. Therefore, the central portion and the vicinity of the side wall of the variable resistance element have different amounts of oxygen in the metal oxide.
Thus, the characteristics of a variable resistance element, particularly the initial breakdown voltage and the resistance value of the variable resistance element in operation vary depending on whether a filament region is generated in the central portion of the variable resistance element or the side wall of the variable resistance element, thereby causing a problem that the performance characteristics of the plurality of variable resistance elements vary. Such a variation in the performance characteristics impairs the stability and reliability of the operation of a semiconductor memory device which uses a variable resistance element. As a solution to cope with such a variation, an additional dimension needs to be added to the design size of the variable resistance element, and consequently miniaturization and increase in capacity of a storage device are prevented.